1. Field of the Invention
The invention relates generally to the field of integrated circuit (IC) device packaging technology, and more particularly to package to package interconnection of IC packages.
2. Background Art
The die-up plastic ball grid array package (PBGA) was first introduced by Motorola and was called an overmolded plastic pad array carriers (OMPAC). See Freyman, and Pennisi, “Overmolded Plastic Pad Array Carriers (OMPAC): A Low Cost, High Interconnect Density IC Packaging Solution for Consumer and Industrial Electronics,” Electronic Components and Technology Conference, IEEE, pp. 176-182, (1991), which is incorporated by reference herein in its entirety.
A PBGA package features a plastic printed circuit board (substrate) typically made of BT (Bismaleimide Triazine) resins or FR4 materials. FIG. 1 shows a conventional PBGA package 100. An integrated circuit (IC) die 102 is attached directly to a top surface of a substrate 110 using a die attach material 106. Wirebonds 114 are used to electrically connect the integrated circuit of IC die 102 to the printed circuit of substrate 110. A matrix of solder balls 108 is mounted on the bottom side of substrate 110. Die 102 and wirebonds 114 are protected from the environment by a plastic molding compound 112. Mold compound 112 encapsulates both die 102 and the wirebond 114, and covers a center region of the top surface of the substrate 110. A periphery of the top surface of substrate 110 is left exposed. FIGS. 2A-2B show plan and side views of PBGA package 100.
A conventional PBGA package, such as package 100, has numerous drawbacks, including: (1) a thick top mold (e.g., mold compound 112) and a large overall package profile height; (2) a small ratio of die size to package size due to a mold cap which must be clamped to the package substrate 110 for molding; and (3) a large package body size.
The Joint Electron Device Engineering Council (JEDEC) has a standard for mold thickness standard for PBGA packages, which is 1.17 mm. With a mold thickness of 1.17 mm, the overall height of a typical PBGA package is in the range of 1.5 mm˜2.5 mm. However, a thinner package is more desirable for many applications, such as hand-held communication devices (cell phones, global positioning devices, watch-size communication devices, etc.), mobile multimedia (video/audio) players, wireless personal area networking devices such as Bluetooth headsets, and flash memory devices and memory cards.
A mold chase with multiple mold caps (mold cavities) is used for mold encapsulation of the JEDEC standard PBGA packages. PBGA package substrates are typically formed in a strip or panel of substrates. Each individual substrate unit in a substrate strip or panel has a corresponding mold cap of the mold chase for molding. Typically, a thermoset molding epoxy is applied in each mold cap, and the mold chase is applied to the substrate strip. In a finished package, the periphery of the top surface of each substrate is exposed as shown in FIGS. 1, 2A, and 2B (i.e., not covered by molding compound 112). Prior to application of the mold compound, both the IC die and the wirebonds must be placed within a mold cavity. Additionally, the IC die and the wirebonds must be kept far enough away from the inner walls of the mold cavity to allow for mold flow and to avoid wire sweeping. Therefore, the size of the IC die is limited by the size of the mold cap (i.e., the mold cavity). Therefore, for a PBGA package, the maximum size of the die must be substantially smaller than the substrate.
Thus, conventional PBGA packages are typically large in body size, ranging from 19 mm×19 mm and above. Large package sizes are undesirable for mobile applications where bulky electronic components make for bulky devices.
To reduce package size, chip scale packages have been developed where the size of the IC die is very close to the size of package. FIGS. 3A and 3B respectively show a perspective view and a cross-sectional view of a fine pitch ball grid array (FBGA) package 300. Similarly to PBGA package 100 described above, in FBGA package 300, a die 102 is mounted to a substrate 110 by a die attach material 106. IC die 102 is electrically connected by a plurality of wire bonds 114 to conductive features (e.g., traces, bond fingers, etc) such as a trace 210 on a top surface of a substrate 110. Mold compound 112 encapsulates die 102, wirebond 114, and the entire top surface of substrate 110. Solder balls 108 of FBGA package 300 may be smaller than solder balls 108 of PBGA package 100, and a smaller ball pitch may be used to space solder balls of FBGA package 300. In addition to the smaller solder balls 108 and smaller ball pitch, thickness of mold compound 112 is reduced to 0.25 mm˜0.7 mm. Molding compound 112 covers the entire top surface of FBGA package 300, enabling an increase in the ratio of the size of die 102 to the size of substrate 110 in FBGA package 300 versus PBGA package 100.
However, while providing some improvement, FBGA packages still suffer from the disadvantages described above with regard to PBGA packages. Thus, what are needed are IC packages that have small size and can accommodate large and complex ICs.